The leader of a group of protesters accused of being members of the far-left movement antifa was sentenced on Tuesday to 100 years in prison after a jury found him and seven other demonstrators guilty of supporting terrorism while taking part in an armed assault last summer against an immigration facility in Alvarado, Texas.

The extraordinary sentence against the protester, Benjamin Song, was only one of the harsh penalties meted out in the case during separate hearings in Federal District Court in Fort Worth by two judges who castigated the defendants for using violence and attacking the democratic process during the protest.

Nine young demonstrators, including Mr. Song, were found guilty in March of an array of charges stemming from the attack on the Immigration and Customs Enforcement facility, which resulted in a police officer being shot in the neck.

Six of the defendants who were convicted of terrorism charges along with Mr. Song were sentenced to between 50 and 70 years in prison. Another, who was found guilty of lesser crimes and was not even present at the protest, was given a term of 30 years in prison. A final defendant is scheduled to be sentenced next month.

The remarkably stiff penalties, issued by Judge Mark T. Pittman and Judge Reed O’Connor, were significantly longer than the lengthiest sentence handed down to any of the more than 1,500 rioters who were prosecuted — and then given clemency — for joining in the attack on the Capitol on Jan. 6, 2021. The most severe sentence faced by a Jan. 6 defendant was the 22-year term given to Enrique Tarrio, the leader of the far-right Proud Boys.

The sentencings in Fort Worth appeared to be a clear signal that, at least in Texas, the courts would deal aggressively with ICE protesters — especially those accused of adhering to the leftist ideology of antifa, a contraction of the word “antifascist.” Activists who have demonstrated against ICE have faced a concerted crackdown from the Trump administration, including 15 people said to be affiliated with two Minnesota antifa groups who were indicted last week on charges of conspiring to impede federal agents during immigration sweeps in the state over the winter.